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OSI Layers Model Explained With Examples

OSI Layers model is a theoretical strategy that describes how information goes to be transmitted over the network. There are a number of layer models obtainable, amongst these following three models are the most well-liked and extremely examined in CCNA Examination.

There are three types of layer model

  1. OSI Reference model
  2. TCP / IP Reference model
  3. Cisco three-layer model

on this introductory half, I defined why OSI Layers model was created. Later I defined the benefits of OSI Layers model, the aim of OSI Layers model and primary providers offered by OSI layers model

OSI  Layers Model

OSI Layers model has seven layers; Application, Presentation, Session, Transport, Network, data link and physical.

OSI Layers Model

 

Application Layer

Application layer offers a method to broadcast and receive data over the network. All applications and utilities that communicate with network fall in this layer. For examples

Browsers :– Mozilla Firefox, Internet Explorer, Google Chrome etc

Email clients: – Outlook Express, Mozilla Thunderbird etc.

FTP clients :– Filezilla, sFTP, vsFTP

Application layer protocols that we should know for exam are following:

SNMP (Simple Network Management Protocol) — Used to manage the connected networking devices.

TFTP (Trivial File Transfer Protocol) — Applied to transfer the files rapidly.

DNS (Domain Naming System) — Applied to translate the name to IP address and vice versa.

DHCP (Dynamic Host Configuration Protocol) — Applied to assign IP address and DNS resources and info automatically to hosts.

Telnet— applied to connect remote devices.

HTTP (Hypertext Transfer Protocol) — Used to browse web pages.

FTP (File Transfer Protocol) — Applied to reliably sends/retrieves files.

SMTP (Simple Mail Transfer Protocol) — Applied to delivers email.

POP3 (Post Office Protocol v.3) — Applied to retrieves email.

NTP (Network Time Protocol) — Used to synchronizes clocks.

 

Presentation layer

Presentation layer prepares the data. It takes data from application layer and marks it with formatting code such as .doc, .jpg, .txt, .avi etc. These file extensions make it easy to realize that particular file is formatted with a particular type of application. Using the formatting presentation, layer also deals with squeezing and encapsulation. It compresses (on sending computer) and decompresses (on receiving computer) the data file. This layer can also encapsulate the data, however, it’s uncommon as this can be done by lower layers more efficiently.Presentation layer also responsible for network data encrypt and decrypt.

OSI Layers Model Presentation Layer

The Session Layer

Session layer handles connections. It establishes, manages, and terminates sessions between two communicating nodes. This layer provides its services to the presentation layer. Session layer also synchronizes dialogue between the presentation layers of the two hosts and handles their data exchange. Just, for example, web servers may have many users communicating with the server at a given time. As a result, keeping track of which user communicates on which path is important and session layer take care of this responsibility accurately.

OSI Layers Model Session Layer

When a device is contacted first, the session layer is responsible for determining which device participating in the communication will transmit at a given time as well as controlling the amount of data that can be sent in a transmission.

Transport Layer

So far CCNA exam is a concern; this is the most important layer to study. It is advisable to paying extra attentions on this layer, as it is strongly tested in the exam.

Transport layer provides following services: –

  • It establishes and controls the connection between two devices.
  • It multiplexes connections that allow multiple applications to concurrently send and receive data.
  • According to drawback data transmission method can be connection-oriented or connectionless.
  • For unreliable data transfer connectionless method is used.
  • The connectionless method uses UDP protocol.
  • For reliable data delivery connection-oriented method is used.
  • A connection-oriented method uses TCP protocol.When Implemented a reliable connection, sequence numbers and acknowledgments (ACKs) are used.
  • Trustworthy connection controls flow through the uses of windowing or acknowledgments.For the exam, purpose takes note of five main functions of the transport layer.

For the exam, purpose remembers five main functions of the transport layer.

  1. Segmentation
  2. Connection management
  3. Reliable and unreliable data delivery
  4. Flow control
  5. Connection multiplexing

OSI Layers Model Transport Layer

Let’s understand these functions in more depth

Segmentation

Segmentation is the process of dividing large data file into smaller files that can be covered by the network. To understand this process thinks about a 700 MB movie that you want to download from the internet. You have 2MBPS internet connection. How will you download a 700MB movie on 2MBPS internet connection?

In this case, the segmentation process is applied. On server transport layer breaks 700MB movie in a smaller size of segments (less than your internet connection speed). Assume that 700Mb movie is divided into 700 segments. Each segment has a file size of 1Mb that your PC can easily download at current connection speed. Now your PC will download 700 small files instead of one large file. So next time when you see download progress bar in the browser, think it about segment receiver progress bar. Once your browser receives all segments from the server, it will pop up a message indicating download is completed. Transport layer at your PC will merge all segments back in a single 700Mb movie file. The end user will never know how a 700Mb movie makes its way through the 2Mbps connection line.

Connection management

Transport layer setup, maintain and tear down connections for session layer. Actual technician of connection is controlled by the transport layer. Transport layer uses two protocols for connection management UDP and TCP.

UDP

UDP is a connectionless protocol. Connection-less transmission is said to be unreliable. Now, don’t get worried about the term “unreliable” this doesn’t mean that the data isn’t going to get its destination; its only means that it isn’t guaranteed to get its destination. Think of your options when you are sending a postcard, put it in the mailbox, and chances are good that it will get where it’s supposed to go but there is no guarantee. There is always a chance of missing in the way. On the other hand, it’s cheap.

TCP

TCP is a connection-oriented protocol. Connection-oriented transmitting is said to be reliable. Think TCP as registry AD facility available in the post office. For this stage service, you have to buy an extra ticket and place a bunch of extra labels on it to track where it is going and it has been. You get a receipt when it is delivered. In this method, you have a guaranteed delivery. All of this costs you more—but it is trustworthy!

Reliability

Reliability means guaranteed data delivery. To ensure delivery of each single segment, a connection-oriented method is used. In this technique before sending any segments three-way handshake operation is done.

Three way handshake process

  1. PC1 sends an SYN single to PC2 indicating that it wants to establish a reliable session.
  2. P2 replies with ACK/SYN signal where ACK is the acknowledgment of PC1’s SYN signal and SYN indicates that PC2 is ready to establish a reliable session.
  3. PC1 replies with ACK signal indicating that is has received SYN signal and session is now fully established.

TCP

Once a connection is established data transmission will be initiated. To supply maximum reliability it includes following functions:-

  • Identify lost packets and resend them
  • Detect packets that returned out of order and reorder them
  • Recognize duplicate packets and drop extra packets
  • Avoid congestion by implementing flow control Flow control
Flow control

The transport layer implements two flow control methods:

  • Ready/not ready signals
  • Windowing
Ready / not ready signals method

In this method, the sender sends data as per its buffer size. The receiver receives data in its buffer. When receivers buffer gets filled, it sends a not ready signal to the sender, so the sender can stop transmitting more segments. Receivers send a ready signal when it becomes ready to receive next segments. This method has two problems.

  • First, the receiver may respond to the sender with a not ready signal only when its buffer fills up. While this message is on its way to the sender, the sender is still sending segments to the receiver, which the receiver will have to drop because its buffer space is full.
  • The second problem with the uses of this method is that once the receiver is ready to receive more segments, it must first send a ready signal to the sender, which must be received before sender can send more segments.
Windowing

In windowing a window size is specified by between sender and receiver. Sender host will wait for an acknowledgment signal after sending the segments equal to the window size. If any packet lost in the way, a receiver will respond with verification for lost packet. The sender will send lost packet again. The window size is automatically set during the three-step handshake process. It can be adjusted all the time throughout the duration of the connection.

Connection Multiplexing/Application Mapping

Connection multiplexing feature allows multiple applications to connect at a time. For example, a server performs a number of functions like email, FTP, DNS, Web service, file service, data service etc. Suppose server has a single IP address, how will it participate in all these different functions for all the hosts that want to connect with it? To make this possible transport layer assigns a unique set of numbers for each connection. These numbers are called port or socket numbers. These port numbers allow multiple applications to send and receive data concurrently.

Port numbers are divided into following ranges by the IANA

Port number Descriptions
0–1023 Well-Known—For common TCP/IP functions and applications
1024–49151 Registered—For applications built by companies
49152–65535 Dynamic/Private—For dynamic connections or unregistered applications

Common TCP and UDP Port Numbers

TCP UDP
FTP 20, 21 DNS 53
Telnet 23 DHCP 67,68
SMTP 25 TFTP 69
DNS 53 NTP 123
HTTP 80 SNMP 161
POP 110
NNTP 119
HTTPS 443

Network Layer

The network layer is accountable to providing logical address known as IP address. Router operates on this layer. Main functions of this layer are following:-

  • Define IP address
  • Find routes based on IP address to reach its destination
  • Connect several data link type together like as Token Ring, Serial, FDDI, Ethernet etc.

OSI Layers Model Network Layer

IP address

IP address a 32-bit long software address which made from two components:

  • Network component: – Defines network segment of the device.
  • Host component :- Defines the specific device on a particular network segment

The subnet mask is used to distinguish between the network component and host component.

IP addresses are divided into five classes.

  • Class A addresses range from 1-126.
  • Class B addresses range from 128-191.
  • Class C addresses range from 192-223.
  • Class D addresses range from 224-239.
  • Class E addresses range from 240-254.

Following addresses have special purpose: –

  • 0 [Zero] is reserved and represents all IP addresses;
  • 127 is a reserved address and it is used for testing, like a loopback on an interface:
  • 255 is a reserved address and it is used for broadcasting purposes.
IP packet

Network layer receives segment from transport layer and wraps it with IP header that is known as datagram.

Datagram

The datagram is just another name of a packet. Network layer use datagram to transfer information between nodes.

Two types of packets are used at the Network layer: data and route updates.

Data packets

Data packets are used to transport the user data across the network. Protocols used by data packets are known as routed protocol. For example IP and IPv6

Route update packets

These packets are used to update the route information within internet work. Routers use these packets. Protocols that send route update packets are called routing protocols; for example RIP, RIPv2, EIGRP, and OSPF

OSI Layers Model Data Link Layer

Main functions of data link layer are

  • Explaining the Media Access Control (MAC) or hardware addresses
  • Defining the physical or hardware topology for connections
  • Specify how the network layer protocol is encapsulated in the data link layer frame
  • Supplying both connectionless and connection-oriented services
  • Explaining hardware (MAC) addresses as well as the communication process that occurs within a media.
MAC Address

MAC address is a 48-bit long layer to address. It is also known as a hardware address. This address is burnt with the device by manufacturing company.

  • The first six hexadecimal digits of a MAC address represent its manufacture company.
  • MAC addresses only need to be unique in a broadcast domain.
  • You can have the same MAC address in different broadcast domains.
Frame

Data link layer receives a packet from network layer and wraps it with layer two Header that is known as a frame. There are two specifications of Ethernet frame.

  1. Ethernet II
  2. 802

Key points to remember:-

  • Ethernet II does not have any sub-layers, while IEEE 802.2/3 has two: LLC and MAC.
  • Ethernet II has a type field instead of a length field (used in 802.3).
  • 802.2 use a SAP or SNAP field to differentiate between encapsulated layer-3 payloads.
  • With a SNAP frame, the SAP fields are set to 0xAA and the type field is used to indicate the layer-3 protocol.
  • 802.2 SAP frame is eight bits in length and only the first six bits are used for identifying upper-layer protocols, which allows up to 64 protocols.
  • 802.2 SNAP frame supports up to 65,536 protocols.

Physical Layer

Physical layer deals with communication media. This layer receives a frame from data link layer and converts them to bits. It loads these bits on actual communication media. Depending on media type these bit values are converted in single. A few use audio tones, while others utilize state transitions—changes in voltage from high to low and low to high.

OSI Layers Model Physical layer

Protocol data unit

A piece of data passed between layers collectively known as PDU (protocol data unit). Layers have different terms to explain it like (segment in transport layer, packet in network layer, frame at data link layer, and signal at physical layer.)

PDU include data file and a consistent body of information attached to data at each successive layer. This information is called header and footer. It includes instructions on how to restore the file to its original state when it receives to the target system.

As a PDU passes through the layers, a header (and footer only on data link layer) is added to the packet with information to the peer layer on the destination system for reconstructing the data on its way back up through the layers of the destination network.

Data Exchange Process

In data exchange process, participating computers work in reverse mode. Layers on receiving computer perform the same task in reverse mode.

The receiving device takes delivery of, handles, and translates the data from the sending device at a specific layer. For example on sending computer presentation layer compress the data, same presentation layer on receiving computer decompress the data.

On sending computer
  • The sending application gets access to the application layer.
  • The application provides data to the presentation layer.
  • Presentation layer format the data as per network requirement and forward it’s to the session layer.
  • Session layer begins the connection and sends the data to the transport layer.
  • Transport layer broke down the large data file in smaller segments and add a header with control information, which are bits designated to describe how to determine whether the data is complete, uncorrupted, in the accurate sequence, and so forth.
  • Segments are forwarded to the network layer. Network layer includes its header, with a logical address and converts it into a packet. Network layer forwards packet to data link layer.
  • Data link layer attaches its header and footer to the packet and converts it into a frame.
  • Frames are forwarded to the physical layers that convert them into signals. These signals are loaded in media.
On receiving computer
  • Physical layer receives signals from media and converts them in frames. Frames are forwarded to the data link layer.
  • Data link layer examines the frame. All tampered frame are dropped here. If the frame is correct, data link layer remove down its header and footer from the frame and hand over the packet to the network layer.
  • Network layer inspections the packet with its own implementations. If it’s found everything fine with the packet, it strips down its header from packet and hand over the segment to transport layer.
  • Transport layer again does the same job. It verifies the segments with its own protocol rules. Only the verified segments are processed. Transport layer removes its header from verified segments and reassembles the segments in data. Data is handed over the session layer.
  • Session layer keeps track of open connection and forwarded the receiving data to the presentation layer.
  • Presentation forms the data in such a way that application layer uses it.
  • Application layer on receiving computer find the appropriate application from the computer and open data within the particular application.
In nutshell

At the sending device, each layer breaks the data down into smaller packets and adds its own header.

  • At the receiving device, each layer strips off the header and builds the data packets into larger packets.
  • Each protocol layer is blind to the headers of any other protocol layer and cannot process them.

TCP/IP Reference Model

TCP/IP protocol model is another popular layer model that describes network standards. For CCNA exam you should know of this model as well. This model has same names of layers as OSI reference model has. Don’t be mix up with the same name, layers at both models have different functionality in each model.

Let’s see how TCP/IP model is different from OSI reference model

TCP/IP vs OSI Layers Model

Application layer:

TCP/IP model merge the functionality of application layer, presentation layer and session layer from OSI model in the single application layer. In TCP/IP model application layer do all activities those are done by upper layers in OSI model. Application layer handles high-level protocols, including data presentation, compression, and dialog manager.

Transport layer:

In TCP/IP model transport layer provides quality of services. TCP protocol is applied for trustworthy data providing. Flow control and error correction methods are used for assured data delivery.

Internet layer:

In TCP/IP model Internet layer provide all the functionality that network layer provides in OSI model. Internet layer is accountable to finding the correct path for datagram [packet].

Network access layer:

Name of this layer may confuse you as OSI model has a layer of the same name. In TCP/IP model network access layer deals with LAN and WAN protocols and all the functionality provided by physical and data link layer in OSI model.

Cisco’s three-layer hierarchical model

Cisco’s three-layers hierarchical model is a set of networking specification provided by Cisco. This model describes which cisco device operates on which layers.

Core Layer

High-speed layer-2 switching infrastructure operates in this layer.

Distribution Layer

Distribution layer stands between access and core layers. Router and layer 3 switch work in this layer.

Access Layer

This layer provides user’s initial access to the network via switches or hubs.

 

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